Systems of the above-mentioned type are known with load-lifting apparatuses which are driven by electric motors and fluidic means. They serve for avoiding too much physical exertion in the case of manually guided movements of all types of loads retained on the load-receiving device. As a result of the load-balancing, the load hangs at a selected height here and can be guided into its intended position with a minimal amount of force being applied. Such a system, which comprises a crane trolley guided on a running-rail structure in at least one horizontal direction, is known, for example, from German Utility Model DE 297 19 865 U1. It may be possible for the load-bearing element of the known load-lifting apparatuses to be flexible and to be wound up on a drum (for example, cable, or chain), or it may also be flexurally rigid.
A load-lifting apparatus with a flexurally rigid load-bearing element is known, for example, from DE 4342715 A1. This laid-open application describes a manually guided manipulator which has a vertical bearing journal about which a horizontally projecting load-bearing arm can be pivoted.
At its end which is directed away from the bearing journal, the load-bearing arm bears a lifting apparatus which has a load-receiving means at its bottom end. The load-bearing arm comprises two sub-arms which are arranged one behind the other and are connected to one another by a joint with a vertical pivot axis and thus form an angled arm. The load-bearing arm has a further angled arm which is formed from two sub-arms and supplements the first to form a changeable parallelogram located in a horizontal plane.
In the case of some known control systems for load-lifting apparatuses, the magnitude of the empty weight and of the load which is to be received has to be preset on a regulator. In order to avoid this disadvantage, it is also possible, as is known from EP 0 733 579 A1, to provide weight-determining means on the load-lifting apparatus.